my $buf; # leave value as undefined so use before a real value is available will result in a warning message.
...;
while (<$input>) {
    ...;
    $buf .= $newletter; # not an error because appending to an undef value is treated the same as appending to an empty string
}

##</code><code>##

    if (defined $buf) { $buf .= $newletter; }
    else { $buf = $newletter; }

##</code><code>##

my $buf = '';
...;
while (<$input>) { ...; $buf .= $newletter; }